One well known form of such apparatus as disclosed in the inventor's prior patents comprises an endless filter belt or cloth guided to provide a substantially horizontal upper run onto which is fed slurry required to be filtered. The belt or cloth is moved forward stepwise so as to progress successive portions of the upper run over a series of vacuum boxes or trays which serve to suck away liquid from the slurry and leave solid from the slurry deposited on the belt or cloth as filter cake. However, other forms of equipment are also known.
In a pipeline leading from the filter means of such apparatus, i.e. from the vacuum boxes or trays in the above-described example, two valves are disposed, which work in opposite manner. Firstly, there is a valve connected to a vacuum source whereby the latter is either connected to the filter means (the vacuum boxes or trays) or disconnected therefrom. Secondly there is a vent valve whereby the pipeline can be connected to atmosphere. One of these may be in a straight-through pipe and the other in a lateral branch. During filtration the vacuum valve is open and the vent valve closed, whereas between filtration periods, e.g. when the belt is being progressed in a belt filter the vent valve is open and the vacuum valve closed.
One of the most important points with regard to the current invention is that these valves are required to change over very frequently, for example every 10 seconds, namely 6 cycles per minute.
A slow acting valve arrangement increases inefficiency by increasing the time during cycling when no filtration takes place.
In contrast, extremely reliable, tight sealing is not essential. In this respect, any vacuum filter has a considerable amount of vacuum leaks, for example through the filter cake and filter cloth, both of which are porous, so slight additional leakage at the valve is of no consequence. Thus, the valves do not have to be of a construction which ensure hermetic sealing and can be of a somewhat "rough and ready" design, particularly since the maximum pressure differential is only 1 atmosphere.
Commercially available valves are not consistent with the aforesaid requirements and are generally unsuitable. They tend to seal hermetically against relatively high pressures, with the seal becoming progressively tighter with time and hence slower in operation. In any event, they are designed for relatively low frequency cycling, e.g. once every 10 minutes, rather than every 10 seconds.